It is well known to attach a disc shaped trigger wheel having a number of peripheral teeth to a crankshaft of an engine. Such a trigger wheel needs to be sufficiently wide to produce a reliable signal.
Conventional practice is to mount a sensor in close proximity to the trigger wheel to sense the passing of each tooth for use by an associated engine management system in order to provide an indication of the rotational position of the crankshaft.
Normally such a trigger wheel is attached to the crankshaft at a position outside of a cylinder block of the engine. Such an arrangement has the disadvantage that the overall package length of the engine is increased which is problematic in many current vehicles.
There is an increasing requirement to produce smaller engines particularly for city cars and hybrid vehicles where the available space to package the engine is limited.
In addition, a front mounted engine represents a hard point so far as a frontal collision is concerned and so it is desirable to keep the engine as small as possible. Therefore any arrangement that can minimize the size and particularly the length of an engine is beneficial.
It is an object of the invention to provide an improved trigger wheel for an engine.
According to a first aspect of the invention there is provided an engine trigger wheel stamped from a single piece of steel comprising a central annular portion and a cylindrical rim portion arranged around an outer periphery of the central annular portion, the cylindrical rim portion having a plurality of circumferentially spaced apart trigger teeth and the central annular portion has at least one aperture formed therein to move a center of mass of the trigger wheel away from an axis of rotation of the trigger wheel wherein there are a number of circumferentially spaced apertures in the cylindrical rim portion and each of the trigger teeth is formed by a land between two adjacent apertures.
Each trigger tooth may extend substantially parallel to an axis about which the trigger wheel rotates in use.
The central annular portion may define a number of circumferentially spaced fixing holes used to fasten the trigger wheel to a crankshaft of an engine.
According to a second aspect of the invention there is provided an engine having a crankshaft located within a cylinder block of the engine, the crankshaft having a number of counterweights to provide dynamic balancing of the crankshaft in use including an end counterweight located in close proximity to one end wall of the cylinder block, the end counterweight having a recess formed therein in which is mounted an engine trigger wheel constructed in accordance with said first aspect of the invention, wherein the trigger wheel is angularly positioned and has an out of balance mass sufficient to compensate for the out of balance effect of the material lost from the end counterweight due to the recess and the cylindrical rim portion of the trigger wheel extends away from said one end wall of the cylinder block.
The trigger wheel may have a number of circumferentially spaced balance apertures and the sizing, spacing and positioning of the balance apertures in the trigger wheel may be arranged to compensate for the material lost from the end counterweight due to the recess.
The depth of the recess in the end counterweight may be substantially equal to the thickness of the central annular portion of the trigger wheel.